Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONStemperatures below design down to 45F. This requires aninternal control that transfers liquid from refrigerant circuit toabsorbent circuit, transfers liquid from absorbent circuit torefrigerant circuit, limits heat input, or a combination. Lowcondenser water temperature decreases energy usage andincreases unit capacity.When the condenser water temperature is too low, theabsorbent contains too much refrigerant and the solutioncrystallizes. A safety control used by some absorption unitssenses when the lithium bromide concentration is getting toohigh with a low cooling water temperature and takes action toavoid crystallization.Absorption chillers are normally used where heat energy isavailable at very low cost such as the exhaust from a steamturbine. They also are used to reduce electric load and thereforepeak electric demand.CHILLER CONTROL REQUIREMENTSBASIC CHILLER CONTROLBasic chiller control is a sensor in the chilled water supplyor return and a controller to provide a control signal to a capacitycontrol actuator. Capacity control is unique to each compressortype. Summarized, the controls for each compressor type are:1. Centrifugal–Controller output operates a pneumatic orelectric actuator to position inlet vanes as a function of thecontrolled temperature. If speed control is available, thecontroller sequences motor or rotor speed with inlet vanes.2. Reciprocating–Controller provides a stepped output tosequence refrigerant solenoid valves, valve unloading,hot gas bypass, or multiple compressors as a function ofcontrolled temperature.3. Screw–Controller operates speed control or a pneumaticor electric actuator to position sliding bypass valve inresponse to temperature input.4. Absorption–Controller output operates a valve tomodulate the steam, hot water, or gas supply to maintaincontrolled temperature.Capacity high limit controls are used on centrifugal and screwcompressors to limit electrical demand during high load periodssuch as morning cool-down. A load limiting control reducesmotor current draw to an adjustable maximum. Capacity ofsome chillers can be reduced to as low as 10 percent.Most chillers with modulating capacity control use proportionalintegralcontrol, and often receive their chilled water setpoint froma BMCS to optimize building energy efficiency.SYSTEM CONTROLS WHICHINFLUENCE THE CHILLERWhatever the configuration of a chilled water system, propercontrol is necessary to meet the overall system requirements.Condenser and chilled water temperatures establish refrigeranthead and energy needed per unit of cooling. Minimumcondenser temperature limits vary for different chiller designs.Condenser temperatures should be maintained as close to theminimum limits as possible to minimize refrigerant head. Actualcondenser water temperature is dependent on outdoor wet bulbtemperatures. Chilled water temperature is dependent on systemdesign and building load.SAFETY CONTROLSWhen an unsafe condition exists, the compressor should stopautomatically. Safety cutout controls may have automatic ormanual reset and include the following:1. High condenser pressure.2. Low refrigerant pressure or temperature.3. Back-up for the low chilled water temperature controller(on some reciprocating chillers).4. High motor temperature.5. Motor overload.6. Low oil pressure.7. Low oil sump temperature.8. High oil sump temperature.9. Chilled water flow interlock.10. Condenser water flow interlock.The preceding are all two-position (on-off) controls. Inaddition, modulating limit controls sensing high condenserpressure or low evaporator pressure-temperature reducecompressor capacity to avoid the safety cutout conditions ofItems 1 and 2.CHILLER/BMCS INTERFACEMost chillers are supplied with micro-processor controllerswith a significant database of safety, operating, monitoring, andsetpoint status and values. The BMCS normally provides controlof the chilled water pump, the cooling tower fans, and the chillersystem (AUTO/OFF commands to the chiller controller). Thechilled water temperature setpoint and on occasions themaximum load setpoint are also dictated by the BMCS.It is desirable for the BMCS to have access to the chillercontrollerdatabase, but due to the cost and complexity of a custominterface to convert the data to a format acceptable to the BMCS,it is seldom done. Adoption of open communication standardprotocols as the ASHRAE BACnet and the Echelon LonMarkwill replace the expensive interfaces with direct interfaces.ENGINEERING MANUAL OF AUTOMATIC CONTROL278